The present invention relates to an apparatus for pulling-up a porous silica glass preform as a precursor of optical fibers from a reactor furnace and a method for pulling-up a porous silica glass preform by using the apparatus. More particularly, the invention relates to an improved apparatus and an efficient method for pulling-up a porous silica glass preform from a reactor furnace free from the problems and disadvantages unavoidable when a porous silica glass preform having a large size is to be pulled up from the reactor furnace in a conventional apparatus and method.
The porous silica glass preform mentioned above as a precursor of silica glass optical fibers is produced mainly by the so-called VAD (vapor phase axial deposition) method, in which a gaseous feed of a volatile silicon-containing compound such as silicon tetrachloride with optional admixture of a dopant such as germanium tetrachloride is subjected to flame hydrolysis in an oxyhydrogen flame to form fine particles of silica which are deposited on the lower end of a vertically held and rotating seed rod made from fused silica glass forming a porous silica body which grows in the axial or vertical direction as the deposition of silica particles proceeds to give an elongated porous rod of silica glass to be subsequently subjected to sintering and vitrification to give a transparent silica glass preform. Therefore, an apparatus is indispensable in the above described process for pulling up the growing porous silica glass preform out of the reactor furnace for the flame hydrolysis in the vertical direction at a velocity corresponding to the growing velocity of the porous body finally to remove the fully grown porous body away from above the reactor furnace to prepare for the next step.
In order to comply with the rapidly growing demand for optical fibers of silica glass in recent years, the porous silica glass preform in the form of a rod prepared by the above described method is also required to have a large size with a length of 2000 mm or even longer. In view of the fact that the fine silica particles formed by the flame hydrolysis in a vertically elongated reactor furnace are deposited on the lower end of a seed rod which in turn is held at the lower end of a suspender rod supported and moved in the vertical direction with rotation by a carrying means or a carriage, it is readily understood that the apparatus for pulling up the porous silica glass preform of a so large length must have a very large height including the lengths of the porous preform as grown, seed rod and suspender rod, the length of which must be sufficiently large to cover the stroke corresponding to the length of the preform. For example, the suspender rod must have a length of about 2500 mm when a 2000 mm long porous preform is to be pulled up from the reactor furnace so that the effective up-and-down stroke of the carriage must be about 4800 mm or longer and the overall height of the preform-manufacturing apparatus sometimes exceeds 9 meters or even larger.
It may be too much to say that an apparatus having a so large height is accompanied by several disadvantages. Firstly, the cost for constructing such a large apparatus is necessarily very high as compared with the cost for a more compact apparatus. Secondly, a serious problem is caused relative to the accuracy of the rotating movement of the suspender rod which is rotated unavoidably with eccentricity when the length thereof is great since the suspender rod is held by the carriage only at the upper part to greatly affect the uniformity of the porous glass preform. Thirdly, the plant house in which the apparatus is installed must be high enough as a matter of course which is built with a very large construction investment and requires a large amount of maintenance costs. In sum, the height of the preform pulling-up apparatus is one of the important determinant factors of the costs for the manufacture of optical fibers because of the large investment therefor and the limitation in the improvement of the productivity.